With the realization of faster telecommunication data rates and an expanding interest in ultrafast chemical and physical phenomena, it has become important to develop techniques that enable simple measurements of optical waveforms with subpicosecond resolution. State-of-the-art oscilloscopes with high-speed photodetectors provide single-shot waveform measurement with 30-ps resolution. Although multiple-shot sampling techniques can achieve few-picosecond resolution, single-shot measurements are necessary to analyze events that are rapidly varying in time, asynchronous, or may occur only once. Further improvements in single-shot resolution are challenging, owing to microelectronic bandwidth limitations. To overcome these limitations, researchers have looked towards all-optical techniques because of the large processing bandwidths that photonics allow. This has generated an explosion of interest in the integration of photonics on standard electronics platforms, which has spawned the field of silicon photonics and promises to enable the next generation of computer processing units and advances in high-bandwidth communications. Several established nonlinear optical techniques exist to measure optical waveforms with few-femtosecond accuracy, but have limited single-shot record lengths of tens of picoseconds and limited update rates.
The phase shift for temporal imaging devices is typically applied using an electro-optical phase modulator, but an alternative scheme can be realized by using a parametric nonlinear wave-mixing process such as sum-frequency generation and difference-frequency generation. This latter technique is called parametric temporal imaging, and consists of wave-mixing with a linearly-chirped pump yielding a converted waveform that is nearly equivalent to the signal waveform with a linear frequency chirp or equivalently a quadratic phase shift as required for a time-lens. Parametric time-lenses have phase-shifts in excess of 100π, which is significantly larger than the 10π maximally possible using an electro-optical phase modulator and therefore greatly extend the applications of temporal imaging systems. A drawback of using the sum-frequency generation and difference-frequency generation second-order nonlinear processes is that only a narrow range of materials possess a second-order nonlinear moment, and the converted waveform is inherently generated at widely different wavelengths from that of the pump or input signal. Waveform measurement based on temporal magnification using difference frequency generation has yielded promising results, including single-shot measurement of ultrafast waveforms with a resolution of less than 900 fs for a simultaneous record length of 100 ps. Waveform measurements based on time-to-frequency conversion using electro-optic modulation have demonstrated a resolution of 3 ps over a 31-ps record length using multiple-shot averaging.
For the success of silicon photonics in these areas, e.g., communications, on-chip optical signal-processing for optical performance monitoring will prove critical. Beyond next generation communications, silicon compatible ultrafast metrology would be of great utility to many fundamental research fields, as evident from the scientific impact that ultrafast measurement techniques continue to make.